The invention pertains to acrylic emulsion pressure-sensitive adhesives which when coated and dried as a film are clear and which resist whitening by the action of hot and ambient temperature water.
A high degree of resistance to water whitening, especially hot water, by pressure-sensitive adhesives (PSAs) is required for some label applications. For example, bottles labeled with clear film PSA labels can be subjected to pasteurization conditions where the label is expected to remain clear through the process. Historically, solvent based adhesives have been used for such demanding applications. Environmental considerations, however, favor elimination of solvents. This and higher coating speeds attainable with emulsion PSAs, favor their use in place of solvent based PSAs.
Emulsion based PSA polymers are made in the presence of surfactants and other water soluble electrolytes such as initiators. In addition, comonomers such as unsaturated carboxylic acids are employed to impart or enhance adhesive properties and tend to be present at the latex-particle interface and remain there during film formation. Films cast from such emulsions are sensitive to water and become cloudy simply when a drop of water is placed on the film. Water whitening is almost always assured when the water is hot, e.g., the temperature of the water is 70xc2x0 C. or higher.
The water resistance of acrylic emulsion polymer films has long been addressed in the art particularly with respect to paint films. The art has observed that the presence of water soluble electrolytes, surfactants and polyelectrolytes such as sodium polymethacrylate cause or enhance water sensitivity (xe2x80x9cThe Applications of Synthetic Resin Emulsionsxe2x80x9d, H. Warson, Ernest Benn Ltd., London, 1972, incorporated herein by reference). It is also well known that surfactants are migratory species that with time move towards and bloom to the surface of films also rendering them water sensitive.
The art has taught that the use of low levels of surfactant, a reduction or elimination in soluble electrolytes and crosslinking between the emulsion particles can all serve to improve or impart water resistance.
U.S. Pat. No. 5,202,375 to Biale, incorporated herein by reference, teaches for this purpose a polymer emulsion that is synthesized with less than about 1% by weight surfactant, has a high surface tension (i.e., greater than 45 dynes/cm) and is made in the presence of high levels of a crosslinking agent. Although the use of such emulsion polymers as PSAs is contemplated, no useful PSA properties are reported. Moreover, with such high levels of crosslinking and low levels of surfactant, poor PSA properties would be expected in addition to significant stability problems during polymerization. The contemplated use is as a water resistant edge sealer for composite boards and plywood. Low swelling of the films by water after long term drying is used to demonstrate water resistance.
European Patent Publication 554,832 to Lu et al., incorporated herein by reference, teaches high solids, moisture resistant PSA emulsion polymers. The emulsion polymer is synthesized in the presence of copolymerizable ionic surfactant and in the presence of a hydrophobic polymeric tackifier with a number average molecular weight ranging from 400 to about 50,000.
There is no mention of water whitening of the PSA film in the ""375 and ""832 patent specifications.
U.S. Pat. No. 4,994,538 to Lee, et al, incorporated herein by reference, teaches a silicone acrylate PSA copolymer having good water whitening resistance synthesized in the presence of an anionic surfactant. Methyl acrylate and vinyl acetate are optionally present.
U.S. Pat. No. 5,286,843 to Wood, incorporated herein by reference, teaches a process for improving the water whitening resistance of a PSA by use of ion exchange to remove water soluble ions. The deionized latex is claimed to provide superior resistance to water whitening.
It has now been found that water whitening can be eliminated or significantly reduced in clear or transparent acrylic emulsion PSA polymer laminate constructions where the PSA polymer is based on alkyl acrylates by including in the monomer charge at least one partially water soluble comonomer and at least one polar comonomer and forming the copolymers in the presence of a nonionic surfactant containing at least about 8 moles (mols) ethylene oxide per mole (mol) and an ionic surfactant containing less than about 10 mols ethylene oxide per mol preferably in which the weight ratio of nonionic to anionic surfactant is at least one-to-one (at least 1:1), and in which the formed emulsion is neutralized with a base to a pH above 7. Polymerization is preferably under conditions which form emulsions having mean xe2x80x9cvolume-averagexe2x80x9d particle diameters (xe2x80x9cPsizexe2x80x9d) of up to about 210 nm.
The adhesive laminate products formed preferably have an opacity less than about 5%, and more preferably less than about 2.5%, as measured using the boiling water resistance test as described herein. Opacity is measured on a laminate of a layer of the PSA exposed to the action of boiling water for 10 minutes (10 min.) or ambient temperature water for 24 hours (24 hr.) while on a biaxially oriented polypropylene film stock and then laminated to a 2 mil polyester film after exposure. Opacity is measured using a spectrocolorimeter.
Independent of the opacity characteristics of the adhesive film, opacity can be stabilized against change with time by the post-polymerization addition to the formed emulsion copolymer of an inorganic electrolyte.
The PSAs of the present invention may be employed as adhesives which as part of a label construction have a xe2x80x9cno label lookxe2x80x9d. The xe2x80x9cno label lookxe2x80x9d is a printed label which, on application to a container, substrate or the like, gives the appearance of direct printing on the container, substrate or the like. To this end, the adhesive constructions remain clear despite the action of moisture under pasteurizing condition.
It has been further found that the tolerance of the coated and dried adhesive film and/or PSA laminate constructions for water surprisingly enables their use under autoclave conditions in that these materials will either remain clear or quickly return to a clear state after autoclaving where solvent based adhesives do not. This makes the adhesives particularly useful for pharmaceutical and medical applications.
The adhesives of the present invention have been further established to have excellent mandrel performance which further enhances their use as adhesives for labels attached to curved surfaces such as bottles and vials used in pharmaceutical and medical applications as well as battery label applications where the adhesive is required to adhere a label to the cylindrical case of a battery. Such applications are also enhanced by established resistance and stability of the adhesives under conditions of high temperature, high humidity and thermal shock.
The ability to absorb moisture without turning white further makes the adhesives particularly attractive to labeling of bottles having wet surfaces, where the wet surface on the bottle is formed by condensation of water on the bottle""s outer surface. Such applications include the labeling of bottles having a low surface energy fatty acid or wax coating to prevent scratching of bottles in route to a labeling station.
Another unique application for the adhesives of the present invention is as adhesives for fleet marking films where the coated and dried adhesive films not only provide resistance to water whitening during label application and in use, but also enable the constructions to conform to rivets and corrugated aluminum sidings and to retain that conformity over time.
There are provided, in accordance with an embodiment of the instant invention, acrylic emulsion PSA polymer compositions which, when coated and dried as a film, are substantially unaffected by the action of hot and ambient temperature water.
In addition to providing a no label look to transparent and clear face stocks where the resistance of the adhesives to whitening of water makes the adhesives uniquely useful for label applications, resistance to the action of steam under autoclaving conditions is also a requirement. The adhesives also have xe2x80x9cwet stickxe2x80x9d adhesive properties, and excellent mandrel performance for vial, bottle and battery label applications. The adhesives are also useful in fleet marketing applications where the adhesive must have good water whitening resistance and the ability to enable the face stock to conform to the contours or rivets and the like, and to retain such adopted shape without tenting.
The acrylic emulsion PSA polymers are comprised of at least one acrylate ester of an alcohol containing at least four carbon atoms, at least one partially water soluble comonomer and at least one polar comonomer. They are formed in the presence of a mixture of nonionic and anionic surfactants preferably provided in a weight ratio of at least 1.5:1 to form emuslions have a Psize of up to about 210 nm, preferably of up to about 165 nm, as measured by laser light scattering, preferably using a Nicomp Instrument spectrometer [Model HN 5-90 equipped with an autocorrelator Model TC-100].
Butylacrylate, 2-ethylhexyl acrylate and mixtures thereof are the preferred alkyl acrylates. Methyl acrylate, methyl methacrylate and mixtures thereof are the preferred partially water soluble comonomers. Acrylic acid is the preferred polar comonomer and is most preferably employed in admixture with another carboxylic acid, preferably methacrylic acid. The formed latex is adjusted to a pH of at least 7. The preferred pH is from about 7.2 to about 9, more preferably from about 7.5 to about 8.
Post-polymerization addition of a water soluble inorganic electrolyte stabilizes opacity of films formed from the emulsion and is applicable to copolymers prepared using other acrylic monomer combination and surfactant systems.
Films cast from such an emulsion when dried exhibit outstanding boiling water and autoclave resistance as demonstrated by low opacity numbers, and in addition, function as wet stick adhesives.
As used herein, polymer particles means particles of a polymer comprising an inner layer and an outer layer surrounding the inner layer. xe2x80x9cInterstitial spacexe2x80x9d as used herein means the space between particles. The inner layer is formed of monomers of low water solubility and the outer layer is formed of monomers of high water solubility.
It is believed in the art that the emulsion particles, on drying, form a film wherein the surfactant and other water soluble species, such as electrolytes, remain in the interstitial space between the particles. The interstitial space provides channels or pathways for the water molecules to diffuse into the polymer film. When water diffuses into the film, the refractive index between the polymer particles and the interstitial space becomes significantly different, leading to light scattering and coloration of the film. This is believed to cause the phenomenon known as xe2x80x9cwater whitening.xe2x80x9d
It was our initial intent to minimize the presence of water sensitive species and to compatibilize the water sensitive species present with the rest of the polymer and thereby prevent the formation of channels for diffusion of water into the polymer film. xe2x80x9cWater sensitivexe2x80x9d species include inorganic species such as potassium persulfate and the like and organic species such as surfactants and the like.
A boiling water resistance test was developed to simulate the effect of pasteurization and as a standard method to determine candidate adhesive polymer opacity. Opacity is the ratio of the reflectance of a sample backed with a white background to that of the sample backed with a black background, multiplied by one hundred, and reported as percent opacity. In the test, a PSA is coated to a level of 20-22 g/m (20-22 g/m2) on a clear 2 mil biaxially oriented polypropylene (BOPP) (Label-Lyte(trademark) 434, Mobil Corp) face stock or backing, dried at 200xc2x0 F. in an oven for 10 min., is cooled, and after cooling, the film face stock or backing is immersed in a beaker of boiling water (95+xc2x0 C.) for 10 min. The PSA coated face stock is then immediately laminated to a clear 2 mil polyester (Mylar(trademark)) film with a plastic squeegee and the opacity of the resultant laminate determined using a spectrocolorimeter (Hunter Lab ColorQuest 45/0). Percentage opacity is used as a measure for boiling water resistance. The lower the number, the greater the boiling water resistance. A laminate of a 2 mil BOPP with an optically clear adhesive to a 2 mil Mylar not subjected to the action of boiling water gave opacities of around 0.4-0.6%. This is the lowest number expected for such an adhesive laminate. An opacity up to about 2.5% was regarded as good. An opacity above 5.0% was regarded as poor for applications requiring a non-water whitening PSA.
A room temperature water (ambient) resistance test was also developed. First, the opacity of the 2 mil BOPP film-adhesive layer-1.5 mil polyester release liner was determined. The polyester liner was removed, and the liner-free PSA coated BOPP face stock immersed in deionized ambient temperature water for 60 min., removed, and re-laminated to the clear 1.5 mil release liner (Hostaphan(trademark) ZSLK, Hoechst Diafoil) and the opacity was remeasured. The difference in percentage opacity before and after immersion was noted. With the good water resistant adhesives, a very small increase in opacity was seen with little or no visual differences before and after immersion in water.
Without being bound by theory, emulsion particles formed in accordance with this invention are envisioned to have xe2x80x9can onion skinxe2x80x9d type structure with the outer layer being more polar than the inner layer and predominantly the result of the copolymerization of an alkyl acrylate with the partially water soluble comonomer and the polar comonomer. The water soluble comonomer is believed to copolymerize in the aqueous phase before being captured by the growing polymer particles.
On neutralization with a base, the outer layer expands to form a hydrophilic zone adjacent to and surrounding the particle inner layer in which hydrophilic species including nonionic and anionic surfactants dissolve or compatibilize. When the level of the water soluble comonomer is too low, the hydrophilic zone is small resulting in exclusion of the hydrophilic species leading to poor resistance to water whitening.
The pH of the latex was found to be critical to obtaining good opacity and therefore resistance to water whitening. We had expected to obtain high water resistance by maintaining the emulsions at a low pH since carboxyl groups would be less ionized. We unexpectedly found, by contrast, that pH had to be above 7, preferably at least about 7.2, to about 9 or more, preferably from about 7.5 to about 8. The bases which may be used for neutralization include weak bases such as ammonia; strong bases, such as sodium hydroxide; organic amines and the like. The presently preferred and most effective base is ammonium hydroxide.
High water resistance is believed to be induced by the increased polarity and size of the hydrophilic layer outside the emulsion particle in which the hydrophilic species can be compatibilized.
The surfactant systems employed during polymerization must be compatible with the whole emulsion polymer, i.e., the predominantly hydrophobic alkyl acrylate inner layer and the hydrophilic copolymer outer layer surrounding the inner layer, and thereby determining water whitening resistance of the coated film product. The surfactant(s) employed during polymerization are also important in stabilizing the particles during polymerization and in storage.
We have found that with anionic surfactants, the level of ethoxylation has to be about 10 moles per mole surfactant or less, preferably 4 moles (ethylene oxide units per molecule) or less for good water whitening resistance. When the average moles of ethylene oxide is too high, opacities tend to be high. It was also observed that even when good, the opacities are not reproducible when an anionic surfactant was the only surfactant. That is, repeated film draw downs from the same emulsion gave desirably low opacities at times and undesirably high opacities at other times.
Other anionic surfactants with higher levels of ethylene oxide (moles per mole) may be used to stabilize the emulsion particles during polymerization or other reasons but do not impart resistance to water whitening.
Functional anionic surfactants include lauryl ether sulfates such as Disponil(trademark) FES-32, Disponil(trademark) FES-993 made by Henkel; salts of sulfated nonyl and octyl phenoxy poly(ethyleneoxy) ethanols such as Aerosol(trademark) NPES-458, Aerosol(trademark) NPES-930 made by Cytek Industries; sulfosuccinates such as Aerosol(trademark) 0T-75, Aerosol(trademark) A-501 made by Cytek Industries; aromatic phosphate ester surfactants such as Rhodafac(trademark) PE-510 made by Rhone Poulenc; copolymerizable surfactants such as sulfated acrylic polyethers such as SAM 211 from PPG Industries and the like. Other anionic surfactants useful for polymerization include dodecyl benzene sulfonate, lauryl sulfate and the like.
It is believed that nonionic surfactants which are more compatible with the acrylic polymers, and which contain about 8 or more mols ethylene oxide per mol surfactant tend to compatibilize in the shell and not be present in the interstitial space.
In order to ensure good stability in the polymerization process and provide particles having good water whitening (boiling water) resistance, it was found necessary to employ at least one nonionic and an anionic surfactant preferably in a total nonionic to anionic surfactant weight ratio of at least about 1:1, preferably at least 1.5:1, more preferably from about 1.5:1 to about 6:1, more preferably from about 1.5:1 to about 4:1.
Among the nonionic surfactants that can be used are octyl and nonylphenol ethoxylates such as the Igepal(trademark) surfactants made by Rhone Poulenc; Polystep(trademark) surfactant made by Stepan Chemical Co. and the like. Copolymerizable nonionic surfactants such as SAM 185N, nonylphenoxypoly(ethyleneoxy)-ethyl acryl(methacry)lates available from Monomer-Polymer Labs and the like may also be used. Ingepal(trademark) Co-887 is the presently preferred nonionic surfactant.
The acrylic emulsion PSA polymers of the invention are comprised of a predominant amount of one or more alkyl acrylate esters of an alcohol containing at least 4 carbon atoms, at least one partially water soluble comonomer, and at least one polar comonomer.
The alkyl acrylate esters serve to control the glass transition temperature (Tg) and the modulus of the formed polymer and are generally present in an amount of from about 55 to about 90% by weight, and preferably from about 55 to about 85% by weight of total monomers.
Alkyl acrylate esters of an alcohol which form the bulk of the monomers of the core and which contain at least 4 carbon atoms in the alkyl group of the alcohol include, among others, n-butyl acrylate, 2-ethyl hexyl acrylate, hexyl acrylate, decyl acrylate, dodecyl acrylate, isooctylacrylate and the like. Methacrylates can also be used. N-butyl acrylate, 2-ethyl hexyl acrylate and mixtures thereof are presently preferred.
A partially water soluble comonomer is required to ensure adequate boiling water or water whitening resistance. It is believed that this comonomer copolymerizes with the polar comonomer and alkyl acrylate ester to form the shell around the emulsion particle core that compatibilizes electrolytes and the anionic surfactant. Partially water soluble comonomers including methyl acrylate, ethyl acrylate, methyl methacrylate, vinyl acetate and the like may used in order to obtain the desired opacity. Methyl acrylate (5.6% by weight soluble in water @45xc2x0 C.) and/or methyl methacrylate (1.5% by weight soluble in water @45xc2x0 C.) are preferred. The partially water soluble monomer content is above about 7% preferably from about 8% to about 25% by weight of total monomers. Levels of about 8 to about 20% by weight of monomers provides optimum opacity and levels above about 10% are generally not required except to modify adhesive properties or glass transition temperature.
In addition to partially water soluble comonomer, a highly polar comonomer is required to be present. The amount of polar comonomer is generally in the range of from about 1 to about 10% by weight of the monomers. The preferred polar comonomers are carboxylic acids containing from 3 to about 5 carbon atoms. Acrylic acid and a mixture of acrylic and methacrylic acids are presently preferred. Among the carboxylic acids, there may be mentioned acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, and the like.
Results to date show that use of a mixture of polar monomers synergistically gives an opacity lower than the same amount of each polar monomer separately. It is well known in the literature that methacrylic acid tends to become buried inside the particle inner layer to a greater extent than acrylic acid during emulsion polymerization. Combination of polar monomers which tend to partition differently between the polymer particles and the aqueous phase during emulsion polymerization may therefore be effectively used to provide an outer layer which solubilizes hydrophilic species.
Other polar comonomers containing carboxylic and hydroxyl groups can also be used. Examples of such polar comonomers could be beta-carboxyethyl acrylate, monomethacryloylethyl succinate, monomethacryloylethyl phthalate, polyethylene(propylene) glycol monoacry(methacry)lates, 2-hydroxyethyl acry(methacry)late, 2-hydroxypropyl acry(methacry)lates and the like.
Other modifying monomers for the acrylic emulsion polymers include one or more diesters of a dicarboxylic acid wherein each ester group of the diester acid independently contains from about 8 to about 16, preferably from about 8 to about 12 carbon atoms. Examples of diesters are di-2-ethyl hexyl maleate, di-2-ethyl hexyl fumarate and mixtures thereof.
Other comonomers, such as styrene, tertiary butyl styrene and the like, may be used to modify the Tg or other copolymer properties.
Other comonomers including N-vinyl caprolactam, hydroxyethyl(propyl) (meth)acrylate, acetoacetoxyethyl (meth)acrylate, and the like, could be used to further enhance adhesion to various surfaces and partitioning between the aqueous and particle phases while other comonomers, such as glycidyl methacrylate or allyl glycidyl ether, can also be used to further enhance high temperature shear properties. Multifunctional acrylates can also be used to cause crosslinking and improve shear properties.
Chemical crosslinkers provided in an amount of up to about 1% by weight, can be effectively used to further increase cohesive strength. These include chromium acetate, zirconium ammonium carbonate, aluminum acetate and the like. Crosslinking may also be achieved using actinic or electron beam radiation.
The copolymers of the instant invention can be stabilized against UV and oxidative degradation by using UV stabilizers and antioxidants.
In addition, fillers, colorants and the like, tackifiers, plasticizers and oils may be added to modify the properties of the acrylic emulsion polymers of this invention.
The monomer proportions are typically adjusted in such a way that the PSA polymers have a Tg of least 10xc2x0 C. below their use temperature and preferably be from about 0xc2x0 C. to about xe2x88x9260xc2x0 C. as measured by a differential scanning calorimeter. Use temperature is the temperature at which a face stock-adhesive laminate is typically applied to a substrate and is normally 25xc2x0 C.
In any event, the polymer, as part of a PSA laminate, has to be designed to convert well, i.e., to slit, diecut, matrix strip, run on printing presses and be dispensed from labeling machinery to clear substrates such as glass bottles.
The acrylic emulsion polymers of this invention are to provide adhesion and tack at room temperature, elevated and low temperatures, to glass and coated glass or other clear surfaces. What is essential is that they provide low opacities by the boiling water resistance test or minimal changes in opacity by the resistance test to room temperature water.
Polymerization can be carried out in the presence of electrolytes such as tetrasodium pyrophosphate to regulate particle size and emulsion viscosity. Electrolytes generally cause particle size to increase with an increase in electrolyte content and this can increase opacity.
Independent of the use of electrolytes during polymerization, it was unexpectedly found that the post-polymerization addition of small levels of an inorganic electrolyte to the latex stabilizes the cast film against increases in opacity with time. This was unexpected because the relevant literature makes clear that water resistance would be adversely affected by the addition of water sensitive species such as electrolytes. However, our results clearly demonstrate the films containing added inorganic electrolytes retained the initial level of percent opacity even after long term aging. The amount of electrolyte to be added will range from about 0.01 to about 0.25, preferably from about 0.03 to about 0.1, percent by weight of the emulsion.
It is presently preferred to employ as added electrolyte alkali metal sulfates, alkaline earth metal sulfates, alkali metal acetates, alkaline earth metal acetates, alkali metal phosphates, alkaline earth metal phosphates and the preferred metal is sodium.
Particle size also controls opacity. Emulsions having mean xe2x80x9cvolume-averagexe2x80x9d particle diameters of up to about 210 nm, preferably of up to about 165 nm, as measured using the Nicomp Instrument spectrometer were found to have good opacities while emulsions having a larger mean volume average particle diameters had greater opacities than required for use as a clear PSA.
Conventional water soluble free radical initiators can be used for polymerization. They include potassium persulfate, ammonium persulfate and sodium persulfate. Other initiators such as water soluble azo initiators (such as Wako V-50 made by Wako Chemicals USA, Inc), redox initiators such as per sulfate/bisulfite catalyst systems can also be used. The results indicate that a small amount of strong acid groups may be necessary in the outer layer of the polymer particles to compatibilize with the strong acid groups of the anionic surfactant. Results also indicate that with low or no per sulfate content, opacities are high. This is contrary to expectation since higher water resistance as indicated by the absence of such water sensitive sulfate groups at the surface was regarded as important to good water whitening resistance.
The emulsion polymers of the instant invention are formed at a solids level of about 45% by weight solids or more preferably from about 50% to about 60% by weight solids and when cast from emulsion onto a face stock (direct coating) or transferred from a release surface to a face stock (transfer coated) are to provide clear coatings at coat weights of about 20 to about 22 g/m2 with low percentage opacities as per the boiling water resistance test.
Percent opacity of the cast film should be about 5% or less, preferably about 2.5% or less, or more preferably about 1.5% or less, as determined in the boiling water resistance test, or should remain essentially unchanged in the room temperature water resistance test. Direct coated film laminates generally give lower opacities than transfer coated film laminates.
Besides providing adhesives for xe2x80x9cno-labelxe2x80x9d look applications, adhesives of this invention have been found to have applications unexpected for emulsion PSAs.
One utility is resistance to autoclaving. Autoclaving is a severe exposure of the labeled vial to steam under pressure and temperatures. Adhesives must resist degradation and resist water whitening. Adhesives of the instant invention were evaluated against sterilizable solvent based acrylic adhesives for autoclaving applications. It was unexpectedly found that the solvent based adhesives would water whiten in an autoclave whereas emulsion adhesives of the instant invention, which were expected to fail, remained clear under the action of autoclaving. This is a surprising result and opposite to what was expected. This makes the adhesives particularly useful as label adhesives for substrates for pharmaceutical and medical applications where autoclaving conditions can be expected. They include containers, vials, syringes and the like where clarity is important.
For the same and other applications, the adhesives were found to have excellent mandrel performance under overlap conditions and excellent adhesion to siliconized glass. Excellent mandrel performance and unique water whitening resistance of adhesive compositions also allows their use as wet stick label adhesives. Wet stick properties are important to the application of labels to substrate such as wine bottles having condensation on their surface as a consequence of being chilled during a filling operation preceding a labeling operation and where condensation of moisture from the atmosphere provides a bottle with water droplets and possibly a film of water. Where such conditions exist the adhesives have been found to take up the moisture of the surface on the bottles without water whitening and adequate adhesion.
The adhesion is excellent and the adhesives may be used for wet stick labeling of bottles that have been coated with a low surface energy coating, such as polyethylene waxes and fatty acid salts, to reduce scratching during conveying of bottles to filling and labeling stations.
Another unique application, in consequence of excellent mandrel performance, is the use of adhesives as a battery label adhesives where the label must vigorously adhere to the case of a battery under low temperature, elevated temperatures, high humidity conditions and under thermal shock conditions.
Another application is as an adhesive for vehicular fleet marking (such as those used to decorate sides of large trucks which have rivets and corrugations) films where the adhesive would stand exposure to the elements for long periods of time and yet be removable from a substrate without excessive adhesive residue and adhere well to rivets so as to avoid tenting. For such applications it is preferred that adhesives have a total acid content of at least about 4.5% to about 10% by weight of the monomers.
In use, the PSA, copolymers of the invention are employed as part of a label laminate of clear or transparent face stock and PSA provided as a self wound tape or label. They are typically applied to clear substrates such as glass or plastic bottles and the like.
In addition to the Boiling Water Resistance and Room Temperature Water Resistance Tests, the following tests were also performed.
Room temperature shear was measured by applying samples of adhesive coated film measuring 1.25 cmxc3x975 cm strips to a 1.25xc3x971.25 cm area of the stainless steel panel. The film is rolled down twice with a 2 kg rubber-clad roller at the rate of 30 cm/min. The panel is then clamped in a vertical position with 500 g weight attached to the exposure end of the strip. The time elapsed before the weight pulls the sample from the panel is measured in minutes
The % gel content was measured by placing an amount of dry adhesive, e.g. 75 mg., in a 10 micron polytetrafluoroethylene membrane filter. The edges of the membrane are sealed to contain the sample and placed in a 20 ml. vial containing tetrahydrofuran. The vial is tumbled for 24 hr., filter taken out and dried at elevated temperature under vacuum. The filter is weighed and the percent insolubles is calculated by the formula:       %    ⁢          xe2x80x83        ⁢    Insolubles    =                    final        ⁢                  xe2x80x83                ⁢        weight        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        sample                    initial        ⁢                  xe2x80x83                ⁢        weight        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        sample              xc3x97    100  
The clear film labels employing the adhesives of this invention are normally applied to glass bottles with minimum application pressure. The adhesive is formulated to wet out rapidly on bottles so as to give a xe2x80x9cno label lookxe2x80x9d within about 24 hr. of application. During bottle making, the glass bottles may be treated with ingredients to prevent the bottles from scratching. The adhesive should also adhere to such treated surfaces and maintain good adhesion characteristics even under severe conditions such as exposure to high water temperatures. This characteristic is determined by what is known as a hot water bottle peel test (HWBP). In this test, a 1xe2x80x3xc3x978xe2x80x3 strip of adhesive coated on BOPP face stock at a coat weight of 20-22 g/m2 is applied to a glass bottle with no applied pressure, allowed to dwell for 20 min. and then immersed in water maintained at 77xc2x0 C. for 0.5 hour. At the end of this period, the bottle is taken out and 90xc2x0 peel immediately measured. Peel is reported in Newtons per meter (N/m).
The following data of Examples 1 to 92 shows various polymerization and post-polymerization conditions on the ability to obtain and maintain water whitening resistance. Attached Table I contains the meaning of certain abbreviations in Tables II to Table IX. Examples 93 to 96 and Controls A to D establish unique application for the pharmaceutical and medical labels, wet stick labels, battery labels and fleet marking labels.